Ethanol has attracted attention as a fuel source alternative to petroleum fuels. The market scale of ethanol is estimated to be 55 million kiloliters for 2010. However, for adopting ethanol as a fuel, it is necessary to purify, by distillation, a crude product obtained from a biomass such as corn, and then to perform dehydration until at least 99.5 wt % is reached.
Conventionally, for the dehydration, an ethanol aqueous solution is concentrated by distillation in a distillation tower until the azeotropic point of the ethanol/water system is nearly reached. Then, dehydration is performed.
In this respect, the present inventors have diligently been developing a dehydration method to which a water separation membrane is applied, as a dehydration method for such an ethanol aqueous solution which is at or close to the azeotropic point. Specifically, in a dehydration method which the present inventors have been developing, a water separation membrane unit is constructed using a water separation membrane, a target liquid such as an ethanol aqueous solution is caused to flow in the water separation membrane unit, and water is sucked through the water separation membrane. Incidentally, a dehydration method according to Patent Literature 1 has been known as a dehydration method which employs a water separation membrane.
However, when the target liquid is caused to flow in the water separation membrane unit, water vaporizes, which is accompanied by removal of latent heat. This causes a tendency that the temperature of the target liquid becomes lower in the flow direction. The temperature decrease has resulted in a drawback that the partial pressure of water is lowered, which in turn makes it impossible to maintain a sufficient driving force for the suction and thus disables suction means from exhibiting the function thereof.
Accordingly, there have been demands to solve such a drawback to prevent the temperature decrease and to maintain the driving force, and also a demand to require no excessively large equipment.
[Patent Literature 1] Japanese Patent Application Publication No. Sho 58-21629